This invention relates to a single screw extruder, a blow molding machine, and an injection molding machine each using a rotary screw which has a screw head of special construction capable of preventing overheating and stagnation of resin at the front of the screw. Although the invention is applicable to an extruder, a blow molding machine and an injection molding machine, for the sake of brevity they are all merely termed a molding machine and the invention will be described in connection with an extruder as a typical example.
The front or outer end of the screw of a prior art extruder has a conical configuration and molten resin extruded by the rotation of the screw is pushed out from the peripheral portion of the metering section of the screw, and then spirally flows as a layer flow along the conical surface of the screw to reach the front of the screw.
The resin in contact with the conical surface of the screw is heated due to its viscosity and rotation of the screw. As the viscous heating continues during this spiral flow around the screw tip, the temperature of the resin continuously increases and reaches its maximum at the center of the screw head. Furthermore, since there is no propelling force generated by the rotation of the screw at the central portion, the resin stays in the central portion.
As described above, with a prior art screw the resin is overheated at the conical surface of the screw head and the molten resin stagnates at the central portion, resulting in the following problems.
1. When the extruder is operated continuously for a long interval, the resin which has stagnated for a long time at the conical portion of the screw deteriorates from the local heating and changes color. A portion of the degraded resin flows into the extruded product and forms streaks or fish eyes having different colors, thus degrading the quality of the extruded products.
2. The temperature of the resin is higher at the central portion than at the peripheral portion due to the heating at the conical portion of the screw head and the stagnation of the molten resin at the central portion. This causes unfavorable influence upon the processing of flat film or sheet with a T die or a coat hanger die. More particularly, the resin temperature distribution in the width direction of the die becomes high toward the center position thereof in the T die having a feed opening at the center as shown in FIGS. 1 and 11 (Prior Art). Due to the difference in the molten resin viscosity caused by such nonuniform temperature distribution of the molten resin, the thickness of the extruded sheet varies in the width direction of the die. Correcting this condition for producing a sheet with uniform thickness is a timely affair.
3. It takes a certain time purge the stagnated resin at the front of the screw when changing the color of the resin so that changing colors take a long time.
For this reason, as shown in FIG. 11 (Prior Art), according to prior art apparatus, a static mixer 26 is provided on the downstream side of the extruder for enhancing the admixing of the molten resin in the radial direction so as to disperse the molten resin flow and to make the resin temperature uniform in the radial direction.
The prior art extruder shown in FIG. 11 (Prior Art) will now be described for the sake of better understanding the instant invention. The extruder shown in FIG. 11 (Prior Art), comprises a hopper 20 for supplying resin raw material and an extruder screw 21 having a screw tip and a cone angle of about 120.degree.. By rotating the screw 21, the resin raw material supplied from hopper 20 is plasticized and conveyed to the downstream side of the extruder, which is toward the left in FIG. 11. The extruder further comprises a heating cylinder 22 containing the screw 21 and provided with an electric heater 23 on the outer periphery. A screen pack 24 and a breaker plate 25 at the front end of the heating cylinder 22 for preventing foreign particles in the molten resin from flowing to the downstream side. A static mixer 26 is provided on the downstream side of the breaker plate 25 for radially dispersing and mixing the resin flow extruded from the front of the screw, thus making the resin temperature uniform.
One proposed solution to A T die 27 is provided for molding the molten resin in a desired configuration the problems described above is to improve the construction of the front of the screw as disclosed in FIGS. 3-5 of Japanese laid open utility model specification No. 8070/1976. Therein, a torpedo shaped member for breaking a resin flow is connected to the front of the screw with a suitable gap between the torpedo shaped member and the inner surface of a heating cylinder. One or more flow paths are formed on the peripheral surface of the torpedo shaped member such that the flow paths are communicated with a single flow passage passing through the center of the torpedo shaped member and opening to the outer end thereof. With this construction, the molten resin flows into a space between the screw and a breaker plate located in the heating cylinder in front of the torpedo shaped member through a gap between it and the heating cylinder. At the same time, the molten resin having the same thermal hysteresis continuously flows out from a central opening of the torpedo shaped member so as to draw surrounding molten resin, thus preventing stagnation of the molten resin.
Where a static mixer is provided for a prior art molding machine as shown in FIG. 11, (Prior Art) the temperature of the molten resin at the front portion of the screw can be made uniform but this is not true for deteriorated resin stagnating at the front end of the screw. Furthermore, resin color can not be changed satisfactory.
In the Japanese specification described above, the resin is also pushed out from the central opening of the screw such that the resin stagnates at the central portion in front of the screw. The resin pushed out from the periphery of the screw and the resin pushed out from the central portion of the screw do not sufficiently, admix which was confirmed by the comparison test (example 4 and FIG. 10 to be described later) regarding color change time and admixing property.
Additionally, the frictional heating of the resin caused by the rotation of the conical surface of the screw can not be eliminated since the resin flows helically along the conical surface of the screw.